Some type of rotary.
For me, the increase in performance is negligable. The 3401 really shines on heavy oxidation, but for your everyday swirls I feel like I can get comparable results from my Porter Cable. Same with the Duetto.
They do offer a smoother running experience, but I can't tell you how many times I just end up putting the PC back into play.
The only time I lose cut on the PC is when the pad loads up. Even using something like Ultimate Compound, it cuts hard enough that after a couple of passes, I turn over the machine, and see that residue build up that's clogged up my pads. This even after cleaning them after every section cycle.
The 3401 and the Rupes are both nice though. I'm considering the next Flex XCE for either the shop, or the mobile rig.